Two ways to prove a British signature
Inside a shared treaty, one slip settles the question and every member honours it. Outside that circle, the receiving country puts the file in front of its own officer before it will trust the signature on it. This route runs on the second method, so the British mark starts the process and the consular stamp closes it.
Guinea-Bissau and the Hague Convention
Guinea-Bissau has not joined the Hague Apostille Convention. A British paper heading to Bissau therefore requires embassy legalisation whatever the Legalisation Office has already added. Britain is a party to that treaty, which is why the stamp is issued here without fuss, but a treaty only removes a step when the country receiving the paper is inside it too.
The British half, arranged from Sheffield
This part never happens in person. A single office covers every British city by post, so there is no counter in Sheffield to attend. The city supplies the raw material instead: the Register Office at the Town Hall for civil entries, a court for an order, a university or employer for anything to do with work.
An apostille from United Kingdom to Guinea-Bissau is best sent as one complete set. Order the certified copies together, post them in one round, and keep a spare at home. On a low-volume route a second attempt costs weeks rather than days, so the aim is to avoid needing one.
Portuguese, and what the file is for
Portuguese is the official language of Guinea-Bissau, which sets it apart from most of its neighbours on this list. A stamped English certificate will still arrive on a Portuguese-speaking desk, so a Portuguese version normally travels with it. Prepare those words after the last mark, so they describe the bundle as it actually stands.
Most files leaving Sheffield for Bissau are about family rather than commerce. Birth and marriage entries carry nationality and inheritance claims. Behind them come police records and degrees for people taking aid, fisheries or medical postings. In both cases the two-stamp path is identical, and only the deadlines and the urgency differ.